At this time there are several processes for reducing the sulfur level of ferrous alloys such as cast-iron melts and steels.
One of these processes consist in introducing into the ferrous alloy, preferably brough to the liquid state by heating to a temperature at least equal to its melting point, a predetermined quantity of a mineral metallic composition such as a carbide, silicide or oxide of calcium, magnesium, the manganese oxide MnO etc., less stable at the temperature of the molten alloy than the sulfide of the metal of this composition, i.e. the sulfide of calcium, magnesium or manganese.
It has also been proposed to introduce into a molten ferrous alloy a metal having the property of forming a sulfide of the metal stable at the temperature of the molten alloy, e.g. magnesium or calcium.
These processes have, however, disadvantages. Thus the processes which require the addition to the melt of metallic compounds of the type of calcium carbide, calcium silicide, or calcium oxide, magnesium oxide or the manganese oxide MnO, do not yield high efficiency of desulfurization (i.e. the reduction in the concentration of sulfur in the alloy to a very low value).
Furthermore, the yield of such processes is low (i.e. it is necessary to use a relatively large quantity of the metallic compound to treat a given quantity of the alloy). Moreover, the process is accompanied by the formation of a large volume of slag or dross.
The processes which utilize the elemental metal such as magnesium and calcium for the desulfurization of a ferrous alloy are dangerous because of the risk of explosion inherent in the high reactivity and volatility of these metals.
It has been proposed to eliminate or attenuate these disadvantages, to utilize these metals not in a pure state, but in the form of alloys-- for example in the form of the alloy Al--Ca--Si--Mn--Fe. The use of such an alloy increases the cost of the process, reduces the yield and limits the efficiency.
In order to improve the properties of cast iron by transforming it into nodular iron, a problem which has been termed "nodularization", it has been suggested to treat the molten cast-iron melt (gray cast iron) by introducing into it a certain quantity of an appropriate metal such as magnesium.
This process has the disadvantage that it leads to the formation of sulfur and eventually magnesium oxide, whose presence unfavorably alters the physical properties of the iron.
To eliminate this disadvantage, it has been proposed (U.S. Pat. No. 2,882,266) to form magnesium in situ in the molten iron by the reduction of magnesium chloride (MgCl.sub.2) with calcium silicide after having first reduced the levels of sulfur and oxygen in the melt. Similarly it has been suggested to form sodium in situ by the reduction of sodium chloride with calcium silicide.
To that effect, the latter patent provides for the successive introduction into the melt of a mixture of at least 30% and at most 50% by weight sodium chloride with at least 50% and at most 70% by weight calcium silicide, and then a mixture of at least 30% and at most 50% by weight magnesium chloride with at least 50% and at most 70% by weight calcium silicide.
However, because of the evaporation, the yield based upon the quantity of salt used is small and the process is further characterized by the evolution of noxious fumes upon the voltalization of sodium chloride and magnesium chloride.
The loss of sodium chloride and magnesium chloride in this manner is a consequence of the fact that NaCl and MgCl.sub.2 do not form a homogeneous liquid phase with calcium silicide at the temperature of the melt.